Conventionally, a steering assist apparatus that assists steering of a vehicle by an assist torque generated by a steering assist motor has been known. The steering assist apparatus includes the steering assist motor, a power converter and a drive control unit. The power converter converts electric power supplied from a direct current (DC) power source and provides the converted electric power to the steering assist motor. The drive control unit controls the power converter. The power converter and the drive control unit are both driven with electric power supplied from the same power source.
In general, a voltage level required to drive the drive control unit is lower than a voltage level of the DC power source. Therefore, a voltage-reducing device is disposed between the DC power source and the drive control unit. The voltage supplied from the DC power source is reduced by the voltage-reducing device and provided to the drive control unit. In particular, such a voltage-reducing device is provided by a DC to DC converter having a switching power supply. For example, JP2007-318920A discloses a switching power supply apparatus that provides output voltages to separate loads through two channels.
Recently, in order to meet the requirement of improving reliability of the steering assist apparatus, “multiplex” or “redundancy” system has been proposed. in particular, it has been proposed to employ multiple power converters. In this case, even if one of or some of the power converters have malfunctions, the power converter that is in a normal condition can continue driving of the steering assist motor.
However, in a case where the steering assist apparatus has only one voltage-reducing device and an operation of the voltage-reducing device is stopped due to an increase in temperature of an element of the voltage-reducing device, the electric power supply to the drive control unit is stopped. As a result, the power converters are stopped. Namely, even if the multiple power converters are employed, the driving of the steering assist motor will be stopped.
It will be assumed a structure of providing voltages outputted from two-channels to a single load in the technology of JP2007-318920A. Further, it is considered to employ such a structure to voltage-reducing devices that outputs voltages to the drive control unit as the single load, thereby to implement multiplication of the voltage-reducing device.
However, a main cause of an abnormality of the voltage-reducing device is an increase in temperature of an element. When two voltage-reducing devices, which substantially have the same heat-resistant property, are operated for the same time period under the same heat dissipation environment, the temperature will increase similarly in the two-voltage-reducing devices. Namely, there is a possibility that, when the temperature of one of the voltage-reducing devices exceeds a limit, the temperature of the other of the voltage-reducing devices is at the same level as the one. Namely, when one of the voltage-reducing devices in an abnormal condition and its operation is stopped, the other of the voltage-reducing devices is simultaneously in an abnormal condition. Therefore, the other of the voltage-reducing devices does not function as a back-up device. As a result, the voltage-reducing devices cannot supply the electric power to the drive control unit, and the operations of the power converters are stopped. In such a case, the steering assist motor cannot generate the steering assist force, and hence a driver feels uncomfortable in steering the vehicle.
Even if the increase in temperature of the voltage-reducing device does not reach the limit and the operation of the voltage,-reducing device is not stopped, when the temperature of the voltage-reducing device increases close to the limit, power loss is likely to increase according to the switching operation. Namely, when the voltage-reducing devices are operated in a condition where the temperature of both of the two voltage-reducing devices is high, efficiency is reduced, resulting in an increase in power consumption.